Glitch compensation in electronic circuits

ABSTRACT

A supply circuit for providing pulses of current has a current source, a reference voltage source for controlling magnitude of the current, and a current switch for controlling whether or not the current passes through a load. Also, there is a switch control signal terminal for controlling the current switch, and glitch compensation elements including at least one capacitance circuit and associated capacitor drive circuit for feeding a variable correcting voltage back to the reference voltage, and a controller to control said variable voltage.

INTRODUCTION Field of the Invention

This invention relates to electronic circuits used for signalprocessing, and in particular to circuit modules intended to provideperiodic pulses of current of a controlled magnitude, such as in adriver circuit for delivery of control signals to an array of LEDs.

Prior Art Discussion

Many circuits are required to deliver pulses of current for definedperiods: One such application is the driving of current to an LED orlaser used in a communications system to deliver coded pulses of lightinto an optical fibre. Typically, a current source, delivers a constantcurrent of magnitude set by a reference voltage, and controls whether ornot the current is delivered to the load. The switch is turned on andoff by a switch control signal.

Optical communication systems often use a form of Light Emitting Diode(LED) to generate an optical signal. This is done by sending a currentinto the LED. The light output is proportional to the current in. It ishighly desirable to have clean square wave outputs from the LED with nooptical distortion (overshoot or undershoot). Optical distortion cancause the next stage optical receiver to experiencepulse-width-distortion (PWD) through incorrect calculation of the peakoptical signal. PWD can lead to signal loss.

Since the LED in an optical system provides a light output proportionalto its current input, a glitch on the voltage source that defines thecurrent into the LED will result in optical distortion. It is thereforedesirable to have a glitch-free voltage source in an optical system toprevent optical overshoot or undershoot.

A well-known problem with circuits of such types is that the suddenstarting and stopping of the flow of current through the load will causesudden changes in the voltage level at the output of the switch andthese changes can disturb the voltages in nearby components. Thesevoltage fluctuations, or “glitches”, can affect the reference voltagesupplied to the current source, and cause it to deliver currents with anincorrect magnitude.

It should be noted that the mechanism by which these ‘glitches’ couplebetween nodes is through what are known as ‘parasitic capacitances’. Ina common MOS device for example, there are well known parasiticcapacitors between the gate, drain, and source nodes. These parasiticcapacitors typically have a proportionality to the size of the devices.For example a 100 μm/0.35 μm NMOS would have approximately ten times theparasitic gate-drain capacitance as a 10 μm/0.35 μm NMOS device.

A known approach to compensating for these glitches employs an invertingamplifier (such as described in Analog Integrated Circuit Design, P339FIG. 8.5, ISBN 0-471-14448-7, David A. Johns, Ken Martin) and acapacitor. These additional components add a correcting voltage onto thereference voltage supplied to the current source. As this correctingvoltage is controlled by the same signal as is controlling the switch,it will approximately match the timing of the voltage changes caused bythe switching action, and the gain of the amplifier and the magnitude ofthe capacitor may be set so as to provide a correction of theappropriate magnitude. However, even with such a circuit, some glitchproblems may remain. The overall supply voltage to the circuit may vary,the characteristics of the output load may vary (for example if it heatsup in operation or the ambient temperature changes) or the desiredreference voltage may be altered by the normal operation of some otherpart of the circuit. Any of these changes may alter the magnitude of theinterference glitch, or the time at which the glitch arrives, and hencereduce the effectiveness of any correction circuit.

Another field in which glitch compensation (or ‘charge’ compensation) iscommon is converters, Digital-To-Analog converters (DACs), orAnalog-To-Digital Converters (ADCs). In order to compensate for a glitchdisturbance, a quantity of charge is injected to the node of interestthrough a compensation capacitor driven by an open loop amplifier orinverter. However, this is not optimum for many applications such asdriving LEDs. It neither exerts adequate control over the chargeapplied, nor suitably accounts for the operating conditions of thecircuit.

One approach to improve the robustness of a voltage source to glitchesis to increase the power used to generate the voltage source. In thecase of using a current mirror as a voltage source this usually meansincreasing the current into the mirror device to increase the bandwidthof the node. However, it is preferred to reduce the power requirementsof circuits, so this is undesirable. It will also be impossible toremove the glitch completely, so further glitch reduction circuitry maybe required.

Another way to improve the robustness of a voltage source to glitches isto filter glitches with large capacitors. The reduction of the glitchwill be proportional to the size of the filter capacitor used. In thecase of integrated circuits, capacitors can require a large siliconarea. However, this increases the cost of circuits, and so isundesirable. Since the glitch reduction is proportional to the capacitorsize then an acceptable capacitor size may be used to make some limitedimprovements in conjunction with further glitch reduction circuitry.

Glitches on a voltage source can also be reduced by using a methodcalled ‘current steering’, such as described in Design Of Analog CMOSIntegrated Circuits, International Edition, P515-520, Behzad Razavi,2001, ISBN 0-07-118839-8.

This method reduces the glitch on the voltage source because it neverturns the current output off. Instead, a complementary switch redirectsthe current to a supply rail or to a dummy load. The main disadvantageof this technique is that the average current output is doubled, becauseinstead of being switched off half the time the current is always on.Also this method does not completely remove the glitch because perfectcurrent steering is impossible, and further glitch reduction circuitrymay still be required.

The invention is directed towards achieving improved glitch compensationespecially for communication applications.

SUMMARY OF THE INVENTION

According to the invention, there is provided a supply circuit forproviding pulses of current, the circuit comprising a current source, areference voltage source for controlling magnitude of current deliveredby the current source, a current switch for controlling whether or notsaid current passes through a load, a switch control signal terminal forcontrolling the current switch, and glitch compensation elementsincluding at least one capacitor and associated drive circuit forfeeding a variable correcting voltage to the reference voltage source,and a controller to control said correcting voltage in response to oneor more parameters of the circuit.

In one embodiment, the glitch compensation elements comprise a pluralityof drive elements, each connected in series with a capacitor for feedinga correcting voltage back to the reference voltage, the number of driveelements to operate at any time being dependant on an output of thecontroller.

In one embodiment, the glitch compensation elements comprise a pluralityof capacitors, and at least one of said capacitors is connected into thereference voltage source or disconnected from it by a switch controlledby the controller.

In one embodiment, a glitch compensation element is configured todisable a capacitor, for example by disabling a drive element.

In another embodiment, the glitch compensation elements comprise atleast one delay element and the controller is configured to time saidfeeding of a correcting voltage by control of said delay elementaccording to a target of correcting voltage charge being coincident intime, and equal in magnitude, and of opposite polarity to aglitch-causing parasitic charge.

In a further embodiment, said delay element comprises a series ofinverters and/or a resistor and capacitor circuit.

A parameter may be supply voltage provided to the circuit and/ortemperature at which the circuit is operating and/or output drivecurrent set for the circuit module and/or voltage detected on the load.

In one embodiment, the controller is configured to respond to more thanone parameter and to combine these parameters according to an algorithm.

In another aspect, the invention provides a drive circuit for acommunication system, comprising a supply circuit of any embodiment. Thedrive circuit may comprise LED drive components driven by the supplycircuit.

Additional Statements

According to the invention, there is provided a circuit for providingpulses of current, the circuit comprising a current source, a referencevoltage source for controlling magnitude of the current, a currentswitch for controlling whether or not the current passes through a load,a switch control signal terminal for controlling the current switch, andglitch compensation elements including at least one variable capacitancecircuit for feeding a correcting voltage back to the reference voltage,and a controller to control said variable capacitor.

In one embodiment, the controller is configured to control saidcapacitance in response to a parameter of the circuit.

In one embodiment, the controller is configured to respond to the supplyvoltage provided to the circuit.

In one embodiment, the controller is configured to respond to thetemperature at which the circuit is operating.

In one embodiment, the controller is configured to respond to the outputdrive current set for the circuit module.

In one embodiment, the controller is configured to respond to voltagedetected on the load.

In one embodiment, the control element is configured to respond to morethan one parameter and to combine these parameters according to aformula or algorithm.

In one embodiment, the glitch compensation elements comprise multipledrive elements, each connected in series with a capacitor for feeding acorrecting voltage back to the reference voltage, the number of driveelements to operate at any time being dependant on an output of thecontroller.

In one embodiment, the glitch control elements comprise a plurality ofcapacitors, and at least one of said capacitors is connected into thecircuit or disconnected from it by a switch controlled by thecontroller.

In one embodiment, a switch element is configured to disable acapacitor, for example by disabling a drive element.

In another aspect, the invention provides a circuit as defined above,implemented as part of an integrated circuit designed to provide a drivecurrent for a light emitting element as part of a communications system.

In another aspect, the invention provides a drive circuit for acommunication system, comprising a circuit as defined above in anyembodiment.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:—

FIG. 1 is a circuit diagram of a glitch compensation circuit of theinvention;

FIG. 2 a circuit diagram of an alternative glitch compensation circuit,in which there are multiple glitch compensation paths which are switchedby a control circuit according to measured parameters;

FIG. 3 is a diagram of an alternative glitch compensation circuit;

FIG. 4 is a plot of output of the circuit of FIG. 3 with and withoutglitch compensation, for comparison.

DESCRIPTION OF THE EMBODIMENTS

A circuit provides current pulses and a delay element, and one or moreglitch compensation elements are made variable and controlled by acontrol element taking into account one or more parameters available tothe control element. In some embodiments, there may be multiple similarelements with the control taking the form of connecting or disconnectingone or more of the similar elements.

In the invention glitch compensation elements vary a correcting chargeon a bank of one or more capacitors, to dynamically apply a correctingvoltage to the reference voltage source in response to a glitchdisturbance. The compensation elements target this charge to be equal inmagnitude, equal in time of application, and of opposite polarity to thecharge from the glitch disturbance, thereby cancelling the disturbance.In various embodiments there are different means of varying the quantityof glitch compensation charge. One mechanism employs a variablecapacitance configured by digital control, and another employs amechanism to enable or disable several fixed capacitors in parallel. Itwill be appreciated there may be many other means of creating a variablecapacitance. Another mechanism to vary the glitch compensation charge isthrough controlling a capacitor drive circuit, by for example, alteringthe maximum and minimum voltages applied by the capacitor drive circuit.

Referring to FIG. 1 a supply circuit 1 has a current source 11, areference voltage 12, a current switch 13, an output load 14, a switchcontrol 15, and an amplifier drive circuit 21. Also, there is a delayelement 31, a variable capacitor 32, and a capacitor controller 33.

The purpose of the delay element 31 is to compensate for delayselsewhere in the circuit, so that the correcting voltage to be appliedby the glitch compensation elements arrives at the same time as theinterference glitch. It should be appreciated that the change of voltageon the load due to the supplied current will take a finite amount oftime, proportional to characteristics of the load and the currentsupplied. This time may vary depending on the load characteristics orthe circuit conditions. If the correcting voltage applied by the glitchcompensation element were to arrive much faster than the glitch causedby the changing voltage on the load it would not initially cancel theglitch, and could in fact act as an undesired glitch itself.

A delay element may be required to ensure that timing of the glitchcompensation elements and the glitch itself coincide more closely. Adelay element may comprise of a chain of inverters, or perhaps simply aresistor and capacitor. It should also be appreciated there may beconditions where the voltage response of the load to the currentsupplied is very fast, for which optimum glitch compensation requiresthat this delay be zero, and thus the delay element 31 may be omitted.It will be appreciated that the drawings show the delay elements 31 inFIG. 1 (and 41 in FIG. 2, as set out in detail below) connected betweenthe switch control 15 and the glitch compensation amplifier 21 or glitchcompensation drive circuit 42, but this will not always be optimum. If,in a particular case, it is found that the natural circuit delays aregreater on the glitch compensation side of the circuit than they are onthe output side of the circuit, it may be appropriate that the delayelement be inserted between the switch control and the current switch.

The capacitor control element 33 measures the supply voltage (V_(dd))supplied to the circuit 1 and outputs a signal which controls thevariable capacitor 32 accordingly. A method of monitoring Vdd is asimple Analog-To-Digital Converter (ADC) as part of the same integratedcircuit.

In the simplest case, FIG. 1 might be reduced to a single drive elementwith a pair of capacitors, one of which is always connected, and theother of which is connected or disconnected as required by a signalswitch controlled by a glitch control element.

It is common practice to design integrated circuits to be suitable foroperating at different voltages, for example at 3.3 Volts or 5.0 Volts,because one or other of these voltages are commonly used in electricalequipment, and it is more economical to have one integrated circuit inmass production that will work at both voltages than to produce smallervolumes of two different integrated circuits each designed for aspecific voltage. In the circuit of FIG. 3 the drive elements arecoupled to the voltage supply Vdd. If the Vdd changes then so does thecorrecting voltage applied (the charge on a capacitor is equal to thevoltage multiplied by the capacitance). So, this scheme of varying thecapacitor can allow the glitch compensation circuit to work optimally atboth voltages.

An alternative supply circuit 40 is shown in FIG. 2. In this circuitthere are a current source 11, a reference voltage 12, a current switch13, an output load 14 and a switch control 15. The circuit 40additionally has a multiplicity of delay elements 41, drive amplifiers42, capacitors 43, and signal switches 44. While the elements of eachtype are similar they need not be identical: For example, it may beappropriate that capacitor 2 should have twice the capacitance ofcapacitor 1, and that capacitor 3 should have double the capacitance ofcapacitor 2. The glitch compensation control circuit 45 operates thesignal switches 44 according to information available to it about thecircuit operating conditions, such as supply voltage or temperature, sothat the appropriate glitch correcting signal is applied at all times.

TABLE 1 Example control of switches 44 by circuit 45 on supply voltagein FIG. 2. Supply Voltage (V) SW1 SW2 SW3 3 CLOSED OPEN OPEN 4 CLOSEDCLOSED OPEN 5 CLOSED CLOSED CLOSED

FIG. 2 shows multiple components of each type in the glitch compensationsection, but it may not be necessary to duplicate them all. For example,it may be possible to use a single delay element, a single driveamplifier and multiple capacitors, each connected if required by aswitch.

A further example is shown in FIG. 3. and comprises a PMOS currentsource device 66, a reference voltage source 12 at the gate of this PMOSdevice for controlling magnitude of the current, a current switchcomprising another PMOS device 67 for controlling whether or not thecurrent passes through a load, a load comprising of a resonant cavityLED 65, a switch control signal terminal 64 for controlling the currentswitch and drive circuits, and glitch compensation elements consistingof three capacitors 61, and three capacitor drive circuits 62 comprisingof logic gates, and a controller 63 for providing an enable signal tothe logic gates. The enable signal to these logic gates would beanalogous to the use of switches 44 in FIG. 2. These logic gates wouldhave their power supplies tied to those of the chip, but if desired adifferent regulated supply could be used, the regulation of this supplyitself being a mechanism whereby the glitch compensation could becontrolled. If we assume in this example the intrinsic parasiticcapacitors of the PMOS devices are of the order of 1 pF, then the threecapacitors may be of a similar order of magnitude. As an example theycould be binary weighted as 1 pF, 0.5 pf, and 0.25 pF to allow 0.25 pFstep adjustments of the compensation capacitance from 0.25 pF up to 1.75pF.

FIG. 4 shows oscilloscope measurements of a working example of anembodiment of the circuit of FIG. 3. Two cases are superimposed on thesame axis. The first case is a measurement of the optical output, afteroptical-to-electrical conversion, of an LED being driven by a currentbeing switched on and off in an embodiment in which the glitchcompensation is disabled. This first case shows considerable overshootof the optical signal due to disturbances of the voltage reference tothe current source. This overshoot can be problematic for a receivertrying to detect an accurate signal level. The second case, superimposedon the first, shows the effect of the glitch compensation circuitryremoving the overshoot from the optical signal, providing a much moredesirable signal to an optical receiver.

It will be appreciated that, apart from the supply voltage, there may beother parameters which affect the nature of the glitch control signalrequired. For example, it is common that, to compensate for thereduction in efficiency of LEDs at higher temperatures, an LED drivecircuit may measure the ambient temperature and alter the referencevoltage supplied to the current source at higher temperatures so as toincrease the current supplied to the LED. Even with a constant supplyvoltage, it may be appropriate to change the glitch compensation circuitat higher temperatures or output currents. The glitch compensationcontrol circuit 45 may respond to two or more inputs, apply anappropriate function to each input, sum the result to achieve acomposite input and modify the glitch control elements according to thiscomposite input.

As an example, if a 100 mA output is switched into a 10 ohm load, thevoltage on the load toggles between 0V and 1V. If, because of the sizeof the elements used, the glitch charge on the voltage source due to theswitching is 3 picocoulombs (pC) then the compensation charge must beequal to this. If the output current is programmed to double to 200 mAthen the glitch charge would also approximately double to 6 pC and thecompensation charge must vary accordingly. If the circuit is operated at3V, and the drive elements switch the compensation capacitance betweenthe supply and ground, a 1 pF capacitance will result in a 3 pCcompensation charge, and will compensate the case of the output currentequal to 100 mA. If the supply now changes to 5V this is no longer thecase, and the correct compensation capacitance will be need to be 0.6 pFto supply the necessary 3 pC. In the case of the output current equal to200 mA rather than 100 mA the correct capacitance for the 3V supply caseis 2 pF, and the correct capacitance for the 5V supply case is 1.2 pF.The invention described can use the supplied information (Vdd supply andoutput current) to calculate the optimum compensation required.

The invention is not limited to the embodiments described but may bevaried in construction and detail.

The invention claimed is:
 1. A supply circuit for providing pulses ofcurrent, said supply circuit comprising: a current source, a referencevoltage source for controlling magnitude of current delivered by saidcurrent source, a current switch for controlling whether or not saidcurrent passes through a load, a switch controller for controlling thecurrent switch, and glitch compensation elements comprising: a pluralityof capacitors configured to apply a correcting charge to the referencevoltage source in response to switching on said current switch; at leastone drive element for delivering correcting charges to said capacitors;and a glitch compensation controller configured to control saidcorrecting charge in response to one or more parameters of the supplycircuit by: enabling or disabling said capacitors, or by controllingsaid drive elements to alter maximum and minimum charge applied by thedrive elements to the capacitors, said correcting charge beingcoincident in time, and equal in magnitude, and of opposite polarity toa glitch-causing parasitic charge.
 2. The supply circuit as claimed inclaim 1, wherein at least one of said capacitors is connected to thereference voltage source or disconnected from it by a capacitor switchand said glitch compensation controller is configured to enable ordisable said capacitors by control of said capacitor switch or switches.3. The supply circuit as claimed in claim 1, wherein the glitchcompensation controller is configured to enable or disable one or moreof said a capacitors by enabling or disabling a drive element.
 4. Thesupply circuit as claimed in claim 1, wherein the glitch compensationelements comprise at least one delay element and the glitch compensationcontroller is configured to time said feeding of a correcting charge bycontrol of said delay element according to a target of the correctingcharge being coincident in time, and equal in magnitude, and of oppositepolarity to a glitch-causing parasitic charge.
 5. The supply circuit asclaimed in claim 1, wherein the glitch compensation elements comprise atleast one delay element and the glitch compensation controller isconfigured to time said feeding of a correcting charge by control ofsaid delay element according to a target of correcting voltage chargebeing coincident in time, and equal in magnitude, and of oppositepolarity to a glitch-causing parasitic charge; and wherein said delayelement comprises a series of inverters and/or a resistor and capacitorcircuit.
 6. The supply circuit as claimed in claim 1, wherein aparameter is supply voltage provided to the circuit.
 7. The supplycircuit as claimed in claim 1, wherein a parameter is temperature atwhich the circuit is operating.
 8. The supply circuit as claimed inclaim 1, wherein a parameter is output drive current set for the circuitmodule.
 9. The supply circuit as claimed in claim 1, wherein a parameteris voltage detected on the load.
 10. A drive circuit for a communicationsystem, comprising a supply circuit, said supply circuit comprising: acurrent source, a reference voltage source for controlling magnitude ofcurrent delivered by said current source; a current switch forcontrolling whether or not said current passes through a load, a switchcontroller for controlling the current switch, and glitch compensationelements comprising: a plurality of capacitors configured to apply acorrecting charge to the reference voltage source in response toswitching on said current switch; at least one drive element fordelivering correcting charges to said capacitors; and a glitchcompensation controller configured to control said correcting charge inresponse to one or more parameters of the supply circuit by: enabling ordisabling said capacitors, or by controlling said drive elements toalter maximum and minimum charge applied by the drive elements to thecapacitors, said correcting charge being coincident in time, and equalin magnitude, and of opposite polarity to a glitch-causing parasiticcharge.
 11. The drive circuit as claimed in claim 10, comprising LEDdrive components driven by the supply circuit.